How Does High Blood Sugar Affect the Brain?
Published on July 22, 2025
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People with diabetes sometimes experience large swings in their blood sugar, which can impact the health and function of the brain.
In the short term, high blood sugar can cause neurological symptoms, ranging from lethargy to trouble focusing and altered mental status, sometimes referred to as "brain fog." In the long term, it may increase your risk of more serious cognitive declines. Other common factors such as high blood pressure and high cholesterol play a role in the relationship between diabetes and brain function as well.
Though diabetes creates a risk of cognitive problems, there are multiple steps people with diabetes can take to protect their brain health.
The Brain and Glucose
“The brain relies on glucose as a source of energy for neurons to function,” says Deena Adimoolam, MD, an endocrinologist at Mount Sinai Health System in New York City.
The brain needs a significant amount of glucose to work optimally. Glen R. Finney, MD, a fellow of the American Academy of Neurology, says the brain is the organ with the highest energy requirement in the whole body.
But if you have diabetes, your blood sugar levels will fluctuate outside of the normal blood sugar range.
“Too low or too high blood glucose can damage many organs and tissues in the body, including the brain,” says Dr. Finney.
The relationship between diabetes and the brain is complex, but chronic struggles with high blood sugar may lead to oxidative stress and nerve damage, with an increased risk of cognitive decline.
Dr. Adimoolam says chronically high blood pressure can also cause damage to the blood vessels of the brain, which could lead to decreased blood flow to the brain, resulting in a stroke and or transient ischemic attack.
Hyperglycemia and Brain Fog
High blood sugar, or hyperglycemia, can impact your ability to think and your overall brain function. You may, for example, experience lethargy or feel less sharp than usual. Major fluctuations in blood sugar may also slow down the brain’s processing speed.
Many use the term “brain fog” to describe this feeling of reduced mental acuity that can accompany hyperglycemia. Brain fog is temporary, but it can be a frustrating experience that includes losing your train of thought, struggling to find the right words, failing to stay focused on everyday tasks, and a general sensation of mental exhaustion.
It's possible to bring down temporary high blood sugar levels with an injection of rapid-acting insulin, if you've been prescribed and trained on its use, or with a quick exercise session. The best way to prevent hyperglycemia brain fog is to take your diabetes medications as prescribed, prioritize regular physical activity, and eat a healthy diet.
Extremely high levels of hyperglycemia can lead to severe complications, such as diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic syndrome (HHNS). Both of these conditions are very serious and can sometimes lead to neurological complications. DKA can cause brain swelling, and HHNS decreases cerebral blood flow, resulting in fatigue, difficulty focusing, vision impairment, and even delirium. These conditions, which usually must be treated in a hospital, can cause long-term problems for the brain. Recurrent episodes of DKA may also contribute to altered cognitive function in some individuals.
In severe cases, DKA or HHNS can lead a person to enter a coma, a deep state of prolonged unconsciousness.
Chronically Elevated Blood Glucose and Cognitive Decline
Blood sugar that remains too high for a prolonged time, a defining symptom diabetes, also increases the risk of cognitive health problems. Elevated blood sugar levels lead to a number of issues that can affect the brain, including oxidative stress, inflammation, and insulin resistance and dysregulation.
“We know that exposure to hyperglycemia over long periods of time is associated with a decline in psychomotor speed and mental efficiency,” says Naomi Chaytor, PhD, a clinical neuropsychologist in the state of Washington.
Everyone with diabetes is at risk for cognitive dysfunction, but the specifics may vary depending on the type of diabetes.
Type 2 diabetes and prediabetes may contribute to accelerated aging of the brain, with older brain age occurring alongside higher A1C levels. Mild cognitive impairment is also common among people with type 2 diabetes worldwide.
Meanwhile, type 1 diabetes in older adults is associated with worsening mental and psychomotor efficiency, as well as declining memory. Higher A1C levels may increase the severity of these declines.
Blood Sugar, Diabetes, and Dementia
There’s a strong connection between diabetes and dementia.
Type 2 diabetes impacts the whole body, including the brain and nervous system. The brain relies on a regulated supply of glucose to function, and diabetes negatively impacts the body’s ability to regulate that blood glucose. Diabetes is understood to be a cause of various cognitive impairments, including dementia. People with type 2 diabetes may also be more prone to dementia than people with type 1 diabetes because of associated risk factors like high blood pressure, high cholesterol, and obesity.
“High blood glucose over time increases the risk of cerebrovascular disease and dementias including Alzheimer's disease,” says Finney.
In addition to vascular dementia, people with type 2 diabetes may be at increased risk for Alzheimer’s disease. In fact, because of the close and complex relationship between the two conditions, some experts refer to the link between Alzheimer’s and defects in insulin signaling in the brain as type 3 diabetes.
“There are many hypotheses for this [connection], including that elevated glucose can lead to the neuropathological changes in the brain that are associated with Alzheimer's disease or that insulin resistance in the brain leads to cognitive decline and impairment,” says Dr. Chaytor. But research is ongoing, with mixed (and sometimes conflicting) results.
How to Keep Your Brain Healthy
Anyone, including people with diabetes, can take steps to protect their brain health. A healthy diet, regular exercise, blood pressure management, and social engagement all play a role in safeguarding cognitive function.
Physical activity may be particularly beneficial for the brain health of people with type 2 diabetes. A combination of resistance training and aerobic exercise may even help boost cognition. At the very least, it seems that regular physical activity helps minimize the brain atrophy that can result from type 2 diabetes.
“Exercise is likely your biggest ally, along with [a] good diet that [minimizes] bad sugars and fats,” says Finney.
Careful blood sugar management may also offer protective benefits for brain health in people with diabetes.
Intensive blood sugar control efforts may reduce the rate of cognitive decline and protect existing cognitive function in older adults. But strict blood sugar goals can be too difficult to manage (and therefore aren’t recommended) for people with severe diabetes complications or cognitive or functional impairments.
“Using medications to keep blood sugar well controlled is important,” adds Finney. “Evidence over the decades suggests that some classes of medications used for diabetes may help some people with Alzheimer's disease, but that’s an area of ongoing research.”
“Ensure that your diabetes is well controlled by visiting your primary care doctor or endocrinologist,” says Adimoolam. “If it's not, then ask your doctor how you might be able to get to your goals.”
The Takeaway
- High blood sugar, or hyperglycemia, can negatively impact brain health in both the short and long term, resulting in complications ranging from lethargy and trouble focusing to severe cognitive impairment.
- There’s a complex relationship between diabetes and brain health, but it’s clear that chronic struggles with high blood sugar may lead to oxidative stress and nerve damage, which increase the risk of cognitive decline.
- People with (and without) diabetes can protect their brain by following a healthy diet, exercising regularly, managing high blood pressure, and staying engaged socially.
- If you have diabetes, work consistently with your healthcare team to manage your condition, keep your blood sugar under control, and reduce its potential impacts on your cognitive health.
Resources We Trust
- Mayo Clinic: 6 Tips to Keep Your Brain Healthy
- Centers for Disease Control and Prevention: Your Brain and Diabetes
- Cleveland Clinic: How to Lower Your Blood Sugar Naturally
- Alzheimer’s Research Association: What You Should Know About Alzheimer’s and Type 3 Diabetes
- British Heart Foundation: Blood Sugar Levels: What Is Normal?
Meet Our Experts
Elise M. Brett, MD
Medical Reviewer
Elise M Brett, MD, is a board-certified adult endocrinologist. She received a bachelor's degree from the University of Michigan and her MD degree from the Icahn School of Medicine at Mount Sinai. She completed her residency training in internal medicine and fellowship in endocrinology and metabolism at The Mount Sinai Hospital. She has been in private practice in Manhattan since 1999.
Dr. Brett practices general endocrinology and diabetes and has additional certification in neck ultrasound and fine-needle aspiration biopsy, which she performs regularly in the office. She is voluntary faculty and associate clinical professor at the Icahn School of Medicine at Mount Sinai. She is a former member of the board of directors of the American Association of Clinical Endocrinology. She has lectured nationally and published book chapters and peer reviewed articles on various topics, including thyroid cancer, neck ultrasound, parathyroid disease, obesity, diabetes, and nutrition support.
Dr. Brett practices general endocrinology and diabetes and has additional certification in neck ultrasound and fine-needle aspiration biopsy, which she performs regularly in the office. She is voluntary faculty and associate clinical professor at the Icahn School of Medicine at Mount Sinai. She is a former member of the board of directors of the American Association of Clinical Endocrinology. She has lectured nationally and published book chapters and peer reviewed articles on various topics, including thyroid cancer, neck ultrasound, parathyroid disease, obesity, diabetes, and nutrition support.
Jessica Freeborn
Author
Jessica Freeborn has worked as a health and wellness freelance writer since 2021. She is passionate about encouraging people to take control of their health and stay informed about the latest medical advancements. She has two nursing degrees and has used her healthcare experience to enhance her writing and research.
As someone with type 1 diabetes, she understands the complexities surrounding diabetes management and wants to provide people with accurate information and dispel misconceptions about diabetes treatment.
EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
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